The legal basis is Terry v. Ohio (1968), in which the Supreme Court allowed police to stop and frisk someone they reasonably suspect is armed and involved in a crime. Departments later turned this narrow exception into a mass tactic.
New York City made it a core strategy. In 2011, the NYPD recorded 685,724 stops. According to the NYCLU, 88 percent of those stopped were innocent of any wrongdoing. Black people made up 53 percent of stops and Latino people 34 percent, while white people were 9 percent.
In Floyd v. City of New York (2013), a federal judge ruled that the city's stop-and-frisk practices violated the Fourth and Fourteenth Amendments and amounted to indirect racial profiling. The court appointed a federal monitor. Stops dropped sharply in later years, yet the monitor has continued to report unconstitutional stops, and similar tactics remain in use across the country.